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Waikato researcher re-writing history of ancient human movement across the Pacific

18 February 2020

University of Waikato Radiocarbon dating
A University of Waikato researcher has accurately dated an ancient site in American Samoa.

A University of Waikato researcher is re-writing the history of ancient human movement across the Pacific, by changing the way radiocarbon dates are interpreted.

Dr Fiona Petchey has been helping archaeologists to track the movements of the prehistoric Lapita people through the Pacific region, from their origins in Papua New Guinea to later settlement in the remote islands off Fiji, Tonga and Samoa and the eventual development of diverse Polynesian cultures.

Using miniscule pieces of shell and a rat bone, her work with a researcher from the University of California, Berkeley, has helped to date the ancient site of To’aga, on Ofu Island in American Samoa, adding more than 400 years to its age.

“Radiocarbon dates from To’aga were first obtained in the 1980s, but problems with these dates left researchers thinking it was only 2300 years old. Our research has demonstrated that the site was settled as early as 2800 years ago,” she said.

Dr. Petchey has also dated the site of Bapot in the Mariana Islands, where the oldest settlements in the Pacific islands have been found. Here, 250 years has been shaved off the date of first contact, now dated to be 3300 years ago.

Dr Fiona Petchey
Dr Fiona Petchey of the University’s Waikato Radiocarbon Dating Laboratory.

While the numbers may not sound a lot, she said it equated to many generations, and her work has helped archaeologists better understand how and why these ancient people moved throughout the Pacific.

“It helps us understand how fast these people came into the Pacific. How many groups there were and from which directions they came.”

She said sea level was falling at that time in the Pacific, so this research could also help to understand how these ancient cultures contended with their own climate challenges.

“What environments did these people meet when they arrived on these small islands, how did they cope with dramatic changes, and can this information help us deal with the impact of rising sea level in our future?”


Her research also raised questions about radiocarbon dating methods currently being used.

“I believe the calibration curve currently being used for shells could be out by 200 to 300 years at certain times in the past, resulting in large errors in calendar age, unless the right corrections are made.”

All living things take up the radioactive isotope C14. When they die, C14 starts to decay away at a regular rate, allowing us to date past events. Calibration curves have been developed to help convert this information into calendar ages.

Traditionally only large charcoal samples were considered reliable but new methods now allow much smaller samples of shell and bone to be measured reliably.

“The chance of doing this 40 years ago when To’aga was first excavated, was almost non-existent. We can now date a single shell or even growth rings within the shell, which provides useful environmental information that charcoal dates do not.”

She said her goal was to provide archaeologists with greater control in order to understand these ancient peoples. As we start planning to teach New Zealand history in our schools, the question of Polynesian origins will be introduced to a wider audience.

“I think it’s something that has been very overdue for quite some time. Hopefully the work I am doing will be useful to gaining a greater understanding of this important part of human history.”

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